The 4-point bending test, prescribed by most standards, is the conventional test used to determine the shear resistance of steel-concrete composite slabs, whereas loading requirements are commonly specified in terms of uniform load in standard codes. This article shows a comparison between the results of these two load arrangements. This comparison has been made through both experimental tests and FEM simulations points of view. Both approaches show that the 4-point load arrangement provides higher shear strength than uniform load. The cause of this difference has been studied by means of a realistic FEM approach focusing on the slip mechanics and the 3D contact forces in the steel-concrete interface: longitudinal (shear), transversal (interlocking) and vertical (clamping). It has been concluded that the distribution of contact forces is different from the one load arrangement to the other, mainly because of differences in the distribution of concrete cracks. This article is the result of cooperation between UPC-BarcelonaTech, where the 4-point bending tests and the FEM models were developed, and AdMaS-BUT where the uniform load tests were carried out by means of a vacuum system. The slabs were made with conventional open-rib trapezoidal profile and the Eurocode-4 provisions have been followed in both testing procedures.